Heat Treatment

Currently, high-strength bolts are available in four performance grades: 8.8, 9.8, 10.9, and 12.9. Automotive standards also define 7T, 8T, 9T, 10T, and 11T. All require heat treatment, which is critical to internal quality, mechanical performance, and ensuring specified tensile strength and yield ratio.

The heat treatment process performance generally includes:

• Hardenability
• Hardening capacity
• Sensitivity to overheating and overburning
• Deformation and cracking tendency
• Oxidation and decarburization sensitivity

Main influencing factors: part structure, technical requirements, material characteristics, raw material quality, and pre-treatment condition.

Hardenability: Ability of steel to form martensite through quenching; core of thread must achieve 90% martensite for 8.8–12.9 grades before tempering.

Hardening Capacity: Maximum hardness after quenching; largely depends on carbon content and bolt size.

Overheating Sensitivity: Grain coarsening during heating; fine-grain steels with high Mn or coarse-grain steels are more sensitive.

Overburning Sensitivity: Grain boundary melting or oxidation during heating; promoted by low-melting impurities.

Deformation: Complex shapes, high L/D ratio, and thin-walled areas increase risk; water-quenched steel > oil-quenched > air-cooled. High-alloy steels resist deformation better.

Cracking: Caused by stress concentration (sharp corners, R<0.6 mm), material brittleness, and impurities; water-quenched steel > oil-quenched > air-cooled.

Oxidation: Surface iron or alloy oxidation risk increases with higher temperature, lower carbon, or active elements.

Decarburization: Carbon loss from surface; elements like Si, Co, Al increase risk. Mo-alloyed steels are more prone to decarburization than W-alloyed steels.

笔记： Oxidation and decarburization often occur together.

Goal: Achieve tempered sorbite or tempered bainite+sorbite to ensure mechanical performance. Core must form martensite after quenching, depending on hardenability.

Adjustments needed due to variation in chemical composition. Entire cross-section must form 90% martensite to avoid quench cracking.

Critical diameters for quench cracking:

• Water quenching: 8–12 mm
• Oil quenching: 20–39 mm

Five factors must be controlled during tempering: material differences, furnace type, forging method, thread type, quenching medium.

Mesh belt furnaces are ideal for heat-treating small- to medium-sized high-strength fasteners due to automation, consistent quality, and ability to process fine-threaded parts.

1. Characteristics

• Intelligent Control: Multi-parameter automation, speed, temperature, carbon potential control, data storage for 10+ years.
• High Quality: Temperature fluctuation ±5℃, uniformity ≤10℃, carbon potential ±0.05%, equipped with probes and controllers, atmosphere Methanol+Propane, carbon potential 0.33–0.45%.

2. Operation Essentials

• Cleaning: Residual oil/gas affects atmosphere causing high CH4/CO2, low CO, excessive carbon black formation.
• Loading Uniformity: Temperature fluctuation in heating zones ≤5–10℃, avoid drops >40–50℃ in zone 1 during feeding.